It would be embarrassing if the person was lying about where they were from and didn't actually understand the language you'd be writing. Besides, why would you answer a question that was asked in English, in something other than English? It doesn't make sense.
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BoltClock's a UnicornApr 15 '13 at 5:41

do you mean 'nobody disallow it but just dont' ??
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CharlieApr 15 '13 at 5:46

Our policy disallows it, except in some very specific cases (mostly the "X language and usage" sites). Comments are more of a gray area, if you ask me, since their content is less essential, so you might get away with posting comments in your native language. They will almost always confuse the other users, though.
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Frédéric HamidiApr 15 '13 at 6:09

Comments are secondary to answers and also constitute as content. Comments shouldn't be in anything others than English as well. Have you ever been to an English forum, looked at a thread you found interesting then discovered 80% of the comments are in a language you can't understand? How annoying is that? We want to avoid that here.
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Second RikudoApr 15 '13 at 6:18

Not sure about "English language required", but it's quite normal to see a non-English post being obliterated in minutes, so I'd assume it is at least de facto. +1 for the suggestion to use chat room.
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nhahtdhApr 15 '13 at 6:01

While using local language, One of the drawback I can see is the other users who doesn't understand the non-english/local language will surely flag those comments/answer which will increase moderator's work to review those posts. The result of such review will 100% be deleting those posts.